Misfiring detection of an internal combustion engine is often required for improving combustion control. When a cylinder of the engine is misfiring, un-burnt gas is exhausted from the engine. Consequently, harmful components in the exhaust gas are increased. This also results in lowering of engine power output and increasing pollution. Furthermore, if the combustion control is achieved in such a manner as to increase an intake air amount with a high step-response during infrequent engine misfiring, an idle speed of the engine can become unstable.
Therefore, the misfiring detection is required to determine when misfiring occurs so that various combustion conditions, such as ignition timing and amount of an intake air, can be controlled advantageously.